Typically, each personal computer card contains a Card Information Storage (CIS) unit which stores information related to the nature of the card, and the information is made available to the host PC which responds appropriately. The information may include the manufacturer of the card, the card type and functionality, and its current consumption.
It is known to supply a plug-in extension to a card to extend its functions; such an extension may for example provide a telephone jack so that the card can operate as an Internet interface. The CIS unit must then indicate to the PC the change in nature of operation of the card, and such information is coded into the CIS unit of the card with which the extension is used. At present each card can be connected only to a single type of extension whose characteristics are coded into the CIS unit of the card.